fix(frontend): index if not exists check shall use table's schema

[xiangjinwu]

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What's changed and what's your intention?

Fixes #15195.

This PR splits the resolve_index_schema part out of gen_create_index_plan, and moves the check_relation_name_duplicated step between them to correctly use the resolved schema.

Note:
According to #5791 there was a similar constraint on sink, which has been lifted after support of create sink as query #6648 (comment).

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[chenzl25]

LGTM! thanks.

[yezizp2012]

LGTM~